Buster Posey signed the largest deal for a player with three or fewer years of MLB service / Matt Kartozian, USA TODAY Sports

by Bob Nightengale, USA TODAY Sports

by Bob Nightengale, USA TODAY Sports

Just one day after the San Francisco Giants locked up their brain trust, they made sure that their best player will be sticking around to make them look even smarter.

The Giants signed defending National League MVP Buster Posey to an eight-year, $159 million contract extension Friday that ties him up through 2021, including a full no-trade clause. It's the largest deal in baseball history for a player with three or fewer years of major-league service, and the second-largest contract for a catcher, trailing only Joe Mauer's eight-year, $184 million deal with the Minnesota Twins.

It comes on the heels of the three-year contract extensions for Giants general manager Brian Sabean and manager Bruce Bochy that were announced Thursday.

Posey, 26, has already led the Giants to two World Series championships in his first four seasons, winning the NL Rookie of the Year in 2010, the MVP award last year when he hit .336 with 24 homers and 103 RBI.

Posey, who already signed a one-year, $8 million contract before the season, will receive a $7 million signing bonus, $12.5 million in 2014, $16.5 million in 2015, $20 million in 2016, and $21.4 million in 2017-2021. It also includes a $22 million salary in 2022 or a $3 million buyout.